Though exercise is a principal physiological stress to the cardiorespiratory system, no consensus has been reached on the control scheme underlying the exercise hyperpnea. We propose to study the dynamic interrelationships among response variables in non-steady-state exercise. Correlation among dynamic interrelations constitutes a powerful tool for testing hypotheses concerning underlying physiological mechanisms. We will employ the analytic techniques of control system identification in studies of human subjects. These techniques include: (1) breath-by-breath computerized analysis of metabolic gas exchange, ventilatory and cardiac variables; (2) the use of newly-developed insertable arterial sensors which allow continuous measurement of blood pH; and, (3) the examination of dynamic interrelations among exercise variables in response to sinusoidal forcing by means of computerized Fourier analytic routines. Evidence is accumulating to support the hypothesis that exercise hyperpnea is closely coupled to carbon dioxide delivery to the lung. We propose to further explore this relation and to seek the underlying mechanism. The sinusoidal work rate studies planned should help to uncover the role of cardiac output, carotid body function and metabolic acidosis in dynamic ventilatory control. Further, the potential role of intrabreath arterial pH fluctuations, mechano- or proprioceptors in the exercising limbs, and the metabolic processes in the exercising muscle in the exercise hyperpnea will be defined during both the transient and steady states.